Users have complained, since the commercial introduction of heat pumps, about heat pumps blowing "cold" air. The problem is multiplied as the outdoor ambient temperature drops since the heat pump runs for longer periods of time and the heat pump's ability to transfer heat is reduced. One solution to the problem is to use a complicated system of thermostats, wires and relays to reduce the indoor airflow as the outdoor ambient drops. This would require a technician to install a thermostat outside of the dwelling, string wires from the outdoor thermostat to the indoor unit, install the relays, and adjust the thermostat to the proper setting.
Another method employed in the field is to use a thermostatic sensor in the supply air duct to adjust the supply air flow lower as the supply air temperature drops. System operation would be adversely affected by the setting of the sensor, the quality of the duct work, and the condition of the air filtering system.
A more desirable means of controlling the air flow is to use an electronic control board with electro-mechanical means to adjust the indoor airflow. As the outdoor ambient drops, the heat pump runs for longer periods of time, so that operating on-time of the heat pump comprises a greater fraction of the total cycle time. The heat pump on-time can be tracked as a fraction of total operating time, or "run time fraction." The run time fraction is inversely related, nearly linearly, to the outdoor ambient temperature. Based on dwelling heat load and the capacity of the heat pump, the heat pump cycles on and off in a predictable pattern. The controller can be programmed to reduce the indoor airflow based on run time because heat pumps are installed based on common equipment sizing practices. When the system run time is greater than the controller's set run time, the indoor airflow is reduced. As the outdoor ambient temperature increases, the controller increases the airflow to ensure efficient and reliable operation of the heat pump.
It is therefore a primary object of the present invention to provide a heat pump system that reduces the "cold blow" effect associated with cold ambient temperatures.
It is a further object of the present invention to reduce the "cold blow" effect by running the blower at lower speeds when ambient temperatures are cold (i.e., below 50 F.).
A still further object of the present invention is to reduce the "cold blow" effect by adjusting the blower when the run time fraction (the ratio of heat pump operating on-time to cycle-time) crosses a threshold value.
Another object of the present invention is to reduce the "cold blow" effect by maintaining the blower when the run time fraction is outside a predetermined range of an immediately previous run time fraction.
These and other objects of the present invention are attained by providing a method of selectively reducing the speed of an indoor blower of a heat pump when ambient temperature drops below a predetermined level in order to reduce the "cold blow" effect of air entering the building, by determining an operating run time fraction of a heating cycle, and reducing the speed of the indoor blower when the run time fraction crosses a predetermined threshold value. To avoid changing the speed of the indoor blower at undesirable times (e.g., when the thermostat has just been adjusted), the current speed of the indoor blower can be maintained (kept from changing) when the run time fraction has significantly changed from the run time fraction of a previous heating cycle.